With some fluids there can be a problem with accurately detecting the level of fluid remaining in a storage vessel. When the fluid is being pumped from the storage vessel, if the pump is allowed to continue operating when the storage vessel is empty, this can result in accelerated wear and damage to pump components, such as seals.
For example, accurate level sensors are not available for measuring the level of a cryogenic fluid in a storage vessel. It is known to employ capacitance-type level sensors for measuring cryogenic fluid levels, but even when operating normally such sensors can be in error by as much as 20 to 25 percent, and periodic re-calibrated is needed or at least desirable to prevent drifts in accuracy. Mobile fuel tanks for vehicles can be a particularly challenging application for level sensors since the movement of the vehicle exerts inertial forces on the stored liquefied gas shifting its location inside the fuel tank. If the storage vessel is a fuel tank, such as one that holds liquefied natural gas for delivering fuel to an engine for a vehicle, the vehicle can be stranded if the fuel tank is emptied while the level sensor inaccurately shows that there is still fuel inside the fuel tank.
It can be desirable to store a gas in liquefied form at cryogenic temperatures to achieve a higher storage density, compared to the same gas stored in the gaseous phase. For example, higher storage density is desirable when the gas is employed as a fuel for a vehicle because the space available to store fuel on board a vehicle is normally limited.
The desired temperature for storing a liquefied gas depends upon the particular gas. For example, at atmospheric pressure, natural gas can be stored in liquefied form at a temperature of minus 160 degrees Celsius, and a lighter gas such as hydrogen can be stored at atmospheric pressure in liquefied form at a temperature of minus 253 degrees Celsius. As with any liquid, the boiling temperature for the liquefied gas can be raised by holding the liquefied gas at a higher pressure. The term “cryogenic temperature” is used herein to describe temperatures less than minus 100 degrees Celsius, at which a given gas can be stored in liquefied form at pressures less than 2 MPa (about 300 psig). To hold a liquefied gas at cryogenic temperatures, the storage vessel defines a thermally insulated cryogen space. Storage vessels for holding liquefied gases are known and a number of methods and associated apparatuses have been developed for removing liquefied gas from such storage vessels. The term “cryogenic fluid” is used herein to describe a fluid that is at a cryogenic temperature.
Accordingly, for some fluids, such as liquefied gases stored at cryogenic temperatures, until a more accurate and reliable level sensor is developed and commercialized, there is a need for an apparatus and method for pumping the stored fluid from a storage vessel and detecting when the storage vessel is nearly empty or actually empty.